This invention relates generally to determining the precise initial volume of a mold cavity of an injection molding machine. More specifically, the invention provides a method for filling the mold cavity with a test substance of a known density and compressibility, monitoring the amount of the test substance which enters the mold cavity, and finally calculating the precise initial volume of the mold cavity. Additionally, the invention includes a molding machine with the structural capability of performing the above-mentioned method, and includes automated operation of the molding machine performing the method through the use of a computer program executing on a computer processing unit.
Typically, an injection molding machine consists of an injection assembly and a mold frame. The injection assembly serves to inject a plasticized material into the mold cavity portion of the mold frame. The injection process exerts significant pressure on the mold cavity, with the injection pressure generally increasing over time. Thus, near the end of the injection cycle, when the mold cavity is nearly full, the injection pressure typically reaches its maximum. At this point, the injection pressure tends to force apart the two sections which form the mold cavity which, of course, is undesirable. To prevent this from happening, a piston assembly is included which exerts an opposite force on the mold cavity thereby maintaining the integrity of the mold cavity.
High pressure molding machines utilize piston assemblies designed so that no amount of force exerted by the injection assembly, tending to force the mold cavity apart, can overcome the force exerted by the piston assembly, tending to hold the mold cavity together. Hence the name "high pressure molding". The enormous force required to accomplish this task adds significantly to the cost, size, and weight of the already massive injection molding machines. The excessive pressure causes high pressure molding machines to experience greater wear and tear, and, correspondingly, greater operation and maintenance costs. Additionally, the force required by the high pressure molding machines materially increases the operational danger of the machines. In the past, the amount of material used to fill the mold cavity of a high pressure molding machine, was simple whatever the injection assembly could inject at maximun pressure. In fact, the high pressure molding process starts with more than enough plastic material to fill the mold cavity and simply packs the mold cavity at maximum injection pressure until full. Packing results in many undesirable effects which are well known in the prior art.
Low pressure molding machines, by contrast, utilize significantly lower piston assembly pressures, thus reducing the cost and size of the machines, and the wear and tear on the machines. However, in order to keep the low pressure injection assemblies from forcing the mold sections of the mold cavity apart, low pressure molders must know the precise amount of plastic needed to fill the mold cavity, and not allow more than that amount to enter the mold cavity. Thus, low pressure injection molders do not pack the mold cavity thereby avoiding the higher pressures associated with packing. The injection pressure of a low pressure molding machine still increases over time, just like high pressure molding machines, however, unlike high pressure molding machines, the injection pressure of low pressure molding machines decreases dramatically at the very end of the injection cycle. Accordingly, low pressure molding is dramatically improved by knowing the precise volume of the mold cavity.
While packing the mold cavity represents one of the problems associated with not knowing the precise volume of the mold cavity, short shots represent another problem. Short shots result when not enough plastic material enters the mold cavity. In the case of complex mold cavities, containing irregularly shaped recessed areas, short shots can result in a failure to completely fill the recessed areas. When de-molding the plastic article from the mold cavity, the partially formed recessed areas may break off in the mold cavity from a lack of structural integrity. The residual plastic left in the mold cavity can cause defects in any subsequently molded parts. Additionally, removing the residual plastic from the mold cavity creates time delays and increases setup costs.
The problems associated with low pressure molding, and the techniques for remedying these problems, like the present method, often appear counterintuitive to high pressure molders. High pressure molders will persist in applying high pressure molding techniques, such as packing the mold cavity, to low pressure machines despite instructions to the contrary. This further complicates matters because many users of low pressure injection molding equipment also use high pressure molding machines.
Conventionally, the most common method of determining the initial amount of plastic material to fill the mold cavity of a low pressure molding machine involves a repetitive guessing process. The first guess always leaves a little room in the mold cavity, to avoid packing. Each successive guess increases the amount of plastic until a satisfactory article is molded. This guessing process wastes time and material, varies greatly based on the experience of the molder and the complexity of the molded article, and runs the aforementioned risks associated with short shots.
Accordingly, an exact, repeatable, and automated method for determining the precise initial amount of material needed to fill the mold cavity proves invaluable to low pressure molding, and enables low pressure molders to substantially eliminate the difficulties discussed hereinabove.